The lord of patches - rainbow patches after rain

I recall an incident of my childhood. After a heavy yet beautiful rainfall I went for ride on my bicycle. While riding I saw a beautiful colored rainbow patch on the road and after observing that patch questions started poping inside my mind. What are they ? Has someone droped his paint bottles? or is it some kind of omen? Being an eight year old child I thought that this is something which is visible only to me, so I never asked my questions to anyone because I thought that they will either laugh or they will think that I am mad. Funny,right? I then soon discovered that it's not just me who see these patches but everyone.

So what are these patches? How are they formed? Why do we seen them after rain? What is the mystery behind them?

IT'S ALL SCIENCE

These patches which we see after a rainy day are not some kind of omen or magic but pure science. It's all due to optics. It's a beautiful combination of oil, water and light.

Some amount of oil i.e lubricants from trucks, cars and bikes are always present on the roads. When it rains the oil droplets get collected and start floating on the water surface (Why?). Light which was first passing through just oil or just water has to pass through a layer of them now i.e light will now pass from a layer of air,oil and water. During this process light will not only undergo reflection but also refraction.

PHYSICS AT WORK

The oil film is thickest at the center of the patch and thinnest at periphery. let L' be the length when light ray travels from the top of the oil film and L'' be the length when light ray travels from the point of intersection of oil and water after getting reflected and refracted to our eyes. L' and L'' are slightly different. If the  difference of L'and L'' is an integral multiple of the wavelength of light then constructive interference will occur, the rays will reinforce each other. Such phenomena won't be observed in the case of destructive interference. 

The sun light has all the colors of the rainbow and each color has a different wavelength. The slight difference between the path length causes constructive interference of the colors. As a result of this we observe some beautiful color patches on the roads after rain. Different bands of oil produces different colors because the thickness decreases from the middle to periphery.

Knowledge Booster-

Why oil floats on water?

In simple words density can be defined as number of molecules per unit area. The density of oil is less that that of water. We can say that the oil is lighter then water. So, as a result oil and water don't g mix, oil floats on water.